1. Field of the Invention
The present invention relates to a network-side apparatus in a communication system as well as a system and a method for assigning time slots by such a network-side apparatus. More specifically, the present invention relates to a method for dynamically assigning, by a network-side apparatus, time slots to terminal-side apparatuses through a shared media over a point to multipoint communication system on the basis of fluctuating traffic.
2. Description of the Related Art
Point to multipoint shared media communication systems comprise a single network-side apparatus connected to a plurality of terminal-side apparatuses. The terminal-side apparatuses share one or more physical media (channels) . In other words, the shared physical medium is passively split among multiple terminal-side apparatuses that share the capacity of that medium. Such systems are expected to provide access networks at low costs. Asynchronous Transfer Mode-Passive Optical Networks (ATM-PONs) are an example of the point to multipoint shared media communication system.
FIG. 1 shows an ATM-PON system comprising four terminal-side apparatuses 100 to 130 and a single network-side apparatus 140. The network-side apparatus 140 in this system is connected to the terminal-side apparatuses 100 to 130 through two kinds of transmission lines in order to allow transmission of upstream signals from the terminal-side apparatuses 100 to 130 to the network-side apparatus 140. More specifically, each of the terminal-side apparatuses 100 to 130 is connected to an optical splitter/combiner 160 through individual transmission lines 150 to 153, respectively. The optical splitter/combiner 160 is connected in turn to the network-side apparatus 140 through a common transmission line (hereinafter, referred to as a “shared transmission line”) 170. In order to avoid a collision of data (hereinafter, referred to as “cells”) on the shared transmission line, the terminal-side apparatuses 100 to 130 each uses time slots assigned thereto by the network-side apparatus 140, while the terminal-side apparatuses 100 to 130 shares one frequency band and the network-side apparatus 140 uses a different frequency band.
There are many possibilities in the choice of bandwidth allocation strategy. Of these, assignment of time slots to the terminal-side apparatuses 100 to 130 on a fixed assigned basis is simple to achieve but it prevents efficient use of the bandwidths for the shared transmission line 170 when the terminal-side apparatuses generate bursty traffic of cells. This is because a fixed number of time slots are equally assigned to all terminal-side apparatuses regardless of fluctuation of traffic demands. Such inefficiency stands out more in systems with highly bursty traffic where each terminal-side apparatus makes communications over a best-effort service such as the Internet.
In order to make efficient use of the shared transmission line 170, the network-side apparatus 140 must assign the time slots dynamically on the basis of traffic conditions to the terminal-side apparatuses 100 to 130. In addition, since the channel capacity of the shared transmission line 170 is shared by a plurality of terminal-side apparatuses 100 to 130, it is also necessary to achieve fair bandwidth assignment to the terminal-side apparatuses 100 to 130.
A method for controlling assignment of time slots over a point to multipoint shared media communication system will be described in conjunction with FIG. 1. The terminal-side apparatus 100 comprises a buffer 101, a queue length notification module 102, and an output control module 103. The buffer 101 is connected to the queue length notification module 102 and the output control module 103. The buffer 101 is also connected to terminals 180 and 181 to store the incoming cells from the terminals. The queue length notification module 102 monitors a queue length (i.e., the number of cells stored in the buffer 101) and supplies information indicative of the queue length to the network-side apparatus 140. The output control module 103 controls transmission of cells from the buffer 101. While not being described individually, it is understood that the terminal-side apparatuses 110, 120, and 130 each comprises a similar configuration to that of the apparatus 100.
The network-side apparatus 140 comprises a dynamic time slot assignment control module 141 for use in determining assignment of the time slots to the terminal-side apparatuses 100 to 130. The system comprises a downstream transmission line 190 in order to allow transmission of downstream control signals, especially information indicative of assignment of the time slots, from the network-side apparatus 140 to the terminal-side apparatuses 100 to 130. While the downstream transmission line 190 illustrated in FIG. 1 connects the network-side apparatus 140 only with the terminal-side apparatus 100, it is apparent that the downstream transmission line 190 also connects the remaining terminal-side apparatuses 110 to 130 with the network-side apparatus 140.
There may be another system without the downstream transmission line 190. In this system, the information indicative of the time slot assignment is transmitted through the shared transmission line 170 and the individual transmission lines 150 to 153, from the network-side apparatus 140 to the terminal-side apparatuses 100 to 130.
With the above-mentioned configurations, cells are transmitted from the terminal-side apparatuses 100 to 130 to the network-side apparatus 140 in the manner described below.
Cells supplied from the terminals 180 and 181 to the buffer 101 are stored in the buffer 101. The queue length notification module 102 monitors the queue length and periodically supplies information indicative of the queue length to the network-side apparatus 140. The information indicative of the queue length is herein referred to as “queue length information”. The dynamic time slot assignment control module 141 in the network-side apparatus 140 determines, in accordance with the queue length information, the number and position of the time slots to be assigned each terminal-side apparatus. The dynamic time slot assignment control module 141 supplies the information indicative of the number and position of the time slots to the output control modules 103 in the terminal-side apparatuses 100 to 130 through the downstream transmission line 190. The information indicative of the number and position of the time slots is herein referred to as “timeslot assignment information” for short.
The output control module 103 in the terminal-side apparatus transmits the cells in the buffer 101 to the network-side apparatus 140 in response to the timeslot assignment information. The cells from the terminal-side apparatuses 100 to 130 are transferred to a station (not shown) through the individual transmission lines 150 to 153, the optical splitter/combiner 160, the shared transmission line 170, the network-side apparatus 140, and the transmission line 191.
The dynamic time slot assignment control module 141 in the network-side apparatus 140 determines whether there is any terminal-side apparatus in which the queue length exceeds the threshold value. If any, the network-side apparatus 140 divides the time slots into N number of slot groups and assigns the time slots on an equal basis to the N number of terminal-side apparatuses (wherein N represents the number of the terminal-side apparatuses in which the queue length exceeds the threshold value) to provide fair distribution of the time slots among the terminal-side apparatuses. The network-side terminal 140 then supplies information indicative of the assigned number of the time slots for each terminal-side apparatus through the downstream transmission line 190. The above-mentioned procedure to assign the time slots dynamically and fairly by the network-side apparatus to the terminal-side apparatuses is disclosed in, for example, Japanese Patent Laid-Open No. 10-242981.
Next, an example of this dynamic assignment will be described referring to FIG. 2. In FIG. 2, the queues in the buffers in the terminal-side apparatuses 100, 110, 120, and 130 have the lengths of 10, 50, 40, and 0, respectively. It is assumed that the threshold value is zero and the total number of the time slots available for the terminal-side apparatuses is 60. As mentioned above, the time slots are assigned only to the terminal-side apparatus(es) where the queue length exceeds the threshold value. Therefore, assignment is made to the terminal-side apparatuses 100, 110, and 120 in the illustrated example. The dynamic time slot assignment control module 141 divides the sixty time slots into three slot groups, twenty for each. Subsequently, dynamic time slot assignment control module 141 assigns the time slots equally to the terminal-side apparatuses 100, 110, and 120. As a result, the terminal-side apparatuses 100, 110, 120, and 130 are assigned with 20, 20, 20, and 0 numbers of the time slots. The hatched bar in FIG. 2 corresponds to the slot group containing equally assigned time slots.
As apparent from the above, the N number of terminal-side apparatuses where the queue length exceeds the threshold value are subject to equal assignment of the available time slots. This allows fair assignment of the time slots. However, it would lead to low bandwidth utilization levels when there is one or more terminal-side apparatuses to which time slots are assigned over their queue length. For example, the terminal-side apparatus 100 is assigned with the twenty time slots while the queue length therein corresponds to ten cells. Such “over-assignment” may deteriorate bandwidth utilization levels on the shared transmission line.
Therefore, an object of the present invention is to provide a system and a method for assigning time slots with which a network-side apparatus can assign time slots dynamically and fairly to terminal-side apparatuses by efficiently using bandwidths of a shared transmission line.
Another object of the present invention is to provide a network-side apparatus applicable to the above-mentioned system and method.